There is an urban myth being created about this election, one which flatters David Cameron unnecessarily and lets Labour cabinet ministers off the hook. This is the idea that it's The TV Debate Wot Won It – or rather wot gave the Liberal Democrats their break, denied the Tories a healthy majority, and killed off Labour. And it matters because it leads to drawing the wrong lessons from this campaign.

Mike Smithson, the betting pundit, has argued convincingly that the Lib Dem surge actually started before the first debate: ICM had them at 27 per cent in a poll whose fieldwork was mostly done before the debate started.

That's not unexpected: the Lib Dems usually get a bounce during the campaign, as they suddenly get more coverage than usual from broadcasters following election guidelines.

So what would have happened if there had been no TV debate? Let's assume the broadcasters did lengthy interviews with the three party leaders separately, and let's assume Clegg performed well (as he did with Jeremy Paxman). He wouldn't have drawn as big an audience and his 'I'm not the other two' line would have been less powerful, but it's reasonable to assume he wouldn't have lost ground as a result.

Let's assume Labour ran the same campaign, hampered by unforced errors and public hostility to their leader. Let's assume the Conservatives also ran the same slightly uncertain campaign, centred on a manifesto idea (the Big Society) that voters didn't quite get. And let's assume the same disgust with conventional politics.

Where would the Lib Dems have ended up then? Maybe in the mid-20s in the polls instead of (briefly) the 30s, and stuck there. Instead of entering the last week at.... um, between 25 per cent (ComRes today) and 28 per cent (YouGov).

It's a difference, but not a huge one. So to say the TV debates changed absolutely everything, rather than dramatising absolutely everything (which they certainly have done) is to underestimate Clegg, whose success reflects more than just being good on telly. It's to ignore the caution with which many voters are moving to Cameron.

And it's to miss the point about Gordon Brown. He didn't 'lose' the debates (despite winning several of the arguments) because he's bad on TV: he lost, to put it bluntly, because some voters dislike him so much that they stop listening when they see him.

So the question for Labour's upper echelons isn't why they got suckered into a debate. It's why they sleepwalked into an election with a leader they knew was unpopular, ducking every chance to replace him.